1. Field of the Invention
The present invention relates to an elongating apparatus and method for a glass preform, and a furnace body for the elongating apparatus. In particular, it relates to an elongating apparatus or the like for performing precise elongating for making optical fibers and the like.
2. Related Background Art
Known as elongating apparatus for glass preforms for optical fibers and the like are those disclosed in Japanese Patent Application Laid-Open Nos. 4-83726 and 5-70165.
FIG. 6 is an overall schematic view of a vertical resistance furnace elongating apparatus of the same type as the apparatus mentioned above, whereas FIGS. 7 and 8 are respectively schematic and sectional views showing the configuration of its furnace body portion.
As shown in FIG. 6, dummy rods 41 and 42 are respectively attached to both ends of a glass preform 4, and the latter passes through a furnace core tube 11 (see FIGS. 7 and 8) within the furnace body 1 while its parts corresponding to the dummy rods 41 and 42 are respectively held by an upper chuck 7 and a lower chuck 8. As shown in FIGS. 7 and 8, the furnace body 1 has a four-layer structure in which, successively from the inside, the furnace core tube 11, a heater 12, a thermal insulator 13, and a furnace body outer shell 14 are disposed.
Returning to FIG. 6, while the glass preform 4 is heated by the heater 12 (see FIG. 7) within the furnace body 1, the lower chuck 8 is moved downward in the arrowed direction faster than that of the upper chuck 7, thereby increasing the distance between the upper chuck 7 and the lower chuck 8. Consequently, the glass preform 4 held by the chucks 7 and 8 via the dummy rods 41 and 42 is elongated, thus yielding an elongated body 5. Here, a taper portion 6, which is a boundary between the glass preform 4 and the elongated body 5, is preferably positioned within the furnace body 1 and more preferably positioned within the furnace core tube 11 shown in FIG. 7.
If the heat applied to the glass preform is uneven in the circumferential direction thereof, the heated glass will yield differences in viscosity distribution within its transverse cross section. In this case, even when the glass preform is elongated with the same tension, the distribution of amount of expansion in the axial direction of the glass preform becomes uneven in the transverse cross section, whereby the elongated body may bend or distort its form. Also, as a result, the outer diameter distribution of the elongated body in the axial direction may become uneven. In order to prevent these problems, it is necessary for the furnace core tube and furnace body to be made as uniform as possible so that no unevenness occurs circumferentially in the heat applied to the glass preform.
Japanese Patent Application Laid-Open No. 56-45843 discloses a technique for keeping the outer diameter of such an elongated body constant in its axial direction (hereinafter simply referred to as "conventional control technique"). This technique performs feedback control such that the outer diameter of the taper portion of the glass preform while being elongated is measured, and the moving speeds of the upper and lower chucks are changed according to thus measured value, so as to keep the outer diameter of the taper portion constant, thereby maintaining a constant outer diameter of the elongated body in the axial direction.